1. Field of the Invention
The present invention relates to a process for preparing hydrocarbons having at least 2 carbon atoms, such as ethylene, ethane and propylene, by bringing a methane-containing gas into contact with an oxygen-containing gas in the presence of a catalyst, and to a catalyst to be used when carrying out this process.
2. Description of the Related Art
Since an olefinic hydrocarbon, especially a lower olefinic hydrocarbon such as ethylene or propylene, has a carbon-to-carbon double bond and a rich reactivity, this olefinic hydrocarbon is an important basic material for petrochemical products. In the United States and Canada, wet natural gas (composed mainly of ethane and higher hydrocarbons) is abundantly available at a low cost, and the olefinic hydrocarbon is mainly prepared by processes using this wet natural gas as the starting material. In Japan and European countries, however, naphtha is used as the starting material and a process for thermally cracking naphtha is mainly adopted for the preparation of olefinic hydrocarbons.
Since the oil crisis of 1973, attempts have been made in many countries to use various materials other than naphtha or wet natural gas, and a process of promise is that for preparing ethylene from a dry natural gas (composed mainly of methane) as the starting material.
The exploitable amount of dry natural gas deposits is comparable to the present state of petroleum deposits, and new deposits are being discovered one after the other: It is estimated that the ultimate exploitable amount may be 200 trillion to 250 trillion cubic meters. This natural gas is broadly distributed all over the world, converse to the uneven distribution of petroleum, but nevertheless, this natural gas is still relatively little utilized.
Under this background, many countries are involved in research into the preparation of ethylene from dry natural gas, and various techniques therefore have been proposed in reports and patent specifications.
For example, G. E. Keller et al [J. of Catalysis, 73, 9 (1982)] reported that an alumina catalyst having an oxide of manganese (Mn) or cadmium (Cd) supported thereon is thought to be effective for the conversion (oxidative coupling), but the conversion of methane is low (lower than 5%) and the selectively for ethylene and ethane is very low (lower than 45%). Further, the temperature required for the reaction is relatively high (800.degree. C). Moreover, after research into the activity of a lithium (Li)/magnesia (MgO) catalyst, prepared by a customary catalyst-preparing process, for the oxidative coupling of methane, Ito et al [J. Am. Chem. Soc., 107, 5062, (1985)] reported that use of a 7 weight-Li/MgO catalyst is most preferable and, at 720.degree. C., the conversion of methane is 38% and the selectivity for ethylene and ethane (hereinafter referred to as "C2-selectivity") is 47%. As a result of similar experiments using magnesia catalysts having various oxides supported thereon, Aika et al [Chem. Lett., 1165 ( 1986)] reported that use of a 15 mole %-Na/MgO catalyst is preferable and that, at 800.degree. C., a C2-selectivity of 57% and a C2-yield of 22.4% are obtained. Furthermore, Japanese Unexamined Patent Publication No. 61-207346 teaches that a catalyst having lead oxide (PbO) or lead oxide and manganese oxide (MnO) supported thereon is preferable, and a methane conversion of 26%, a C2-selectivity of 41%, and a C2-yield of 10.7% are obtained at 750.degree. C. If the catalytic activities disclosed in the foregoing literature references and patent specifications are expressed in terms of the spacetime yield of ethylene and ethane (hereinafter referred to as "C2-STY"), it is seen that, where W/F&gt;0.5 g.h/l (W stands for the weight of the catalyst and F stands for the flow rate of the circulated gas), the C2-STY is lower than 3 millimole/g.h, although the reaction temperature is relatively high, and thus the catalytic performance is not satisfactory.
As a recently reported catalyst giving a relatively high C2-STY, there can be mentioned a lanthanum aluminate (LaAlO.sub.3) type catalyst proposed by Imai et al [J. Chem. Soc. Commun. (1986)], which gives a C2-STY of 8.31 millimole/g.h (710.degree. C., W/F=0.125 g.h/l); a 20 weight %-PbO/MgO catalyst disclosed in Japanese Unexamined Patent Publication No. 61-165341, which gives a C2-STY of 10.3 millimole/g.h (717.degree. C., W/F=0.072 g.h/l); a 20 weight %-PbO/MgO catalyst proposed by Asami et al [Chem. Lett., 1233 (1986)], which gives a C2-STY of 11.1 millimole/g.h (760.degree. C., W/F=0.0446 g.h/l); and, a bismuth oxide/potassium carbonate/alumina catalyst (7 weight %-Bi.sub.2 O.sub.3 weight %-K.sub.2 CO.sub.3 /.gamma.-Al.sub.2 O.sub.3) proposed by I. T. A. Emesh et al [J. Phys. Chem., 90, 4785 (1986)], which gives a C2-STY of 90.0 millimole/g.h (700.degree. C., W/F=0.00292 g.h/l). In each of these catalysts, however, the conversion of methane is low (lower than 25%) and the C2-yield is low (lower than 13%), and therefore, the C2-STY is lower than 100 millimole/g.h.
The highest C2-STY heretofore reported is 86 to 2980 millimole/g.h (750.degree. C., W/F =0.000223 to 0.0891 g.h/l) given by a samarium oxide (Sm.sub.2 O.sub.3) catalyst proposed by Otsuka et al [Chem. Lett., 483 (1987)]. In this catalyst, however, the conversion of methane is low (lower than 18%) and the C2-yield is low (lower than 11%), and especially at the maximum C2-STY value, the conversion of methane is about 5.5% and the C2-yield is about 3.4%, which are both very low. Furthermore, the Sm.sub.2 O.sub.3 of this catalyst is a rare earth element oxide, which is very difficult to find and therefore exists in only very small amounts and thus is very expensive. Accordingly, the preparation of ethylene on an industrial scale by using this Sm.sub.2 O.sub.3 catalyst is very difficult.